The carbon and hydrogen stable isotope ratios of methane and carbon dioxide from Ellergower Moss, a raised peat bog in SW Scotland, were measured to characterize the isotopic composition of peatland gas flux that is not subject to near-surface bacterial oxidation. Two profiles of gas samples, collected in situ under pools at 1 m intervals from the surface to a depth of 5 m, gave mean isotope signatures of &dgr;13C(CH4)=-76¿3?(n=22), &dgr;13C(CO2)=3.8¿5.3?(n=22), and &dgr;D(CH4)=-294¿39?(n=21). The profiles revealed two isotopically distinct methane reservoirs within Ellergower Moss, with a boundary layer at 2--2.5 m. The zonation appears to be independent of methanogenic pathway and of changes in &dgr;13C or &dgr;D of bulk substrate, but there is a positive correlation between an increase in &dgr;D(CH4) and reported increasing partial pressure of methane with depth (r=0.943 and n=18, >99.9% significant). To reflect C and H cycling within the peat and gas storage mechanisms, we propose the zones can be termed dynamic (0--2 m) and static (from 3--5 m). The &dgr;D(CH4) profiles exhibit the zonation and boundary most clearly, with a 52¿18? difference between methane in the dynamic zone (-332¿17?) and that in the static zone (-260¿9?). Only if the static zone is breached, will its methane be released to the atmosphere; thus unoxidized peatland flux to the atmosphere will be similar in composition to that produced in the dynamic reservoir, where &dgr;13C(CH4)=-76¿4? and &dgr;D(CH4)=-332¿17?. ¿ 1999 American Geophysical Union